EKGs & Cardiac Contraction

Electrocardiography

Electrocardiography is a non-invasive technique used to record the electrical activity of the heart from the surface of the skin. The resulting record, called an electrocardiogram (ECG or EKG), provides valuable information about the heart's function and rhythm.

• What is measured: The EKG measures extracellular current flow generated by large groups of cardiac cells, not individual cell voltages.

• Voltage Range: The signals are in the millivolt (mV) range.

• Ohm's Law: The relationship between voltage, current, and resistance is described by Ohm's Law: where is voltage, is current, and is resistance.

• Sum of Voltages: The EKG records the sum of external voltages from many cardiac cells.

Key Terms:

• Action Potential: A rapid change in membrane potential that propagates along the cell membrane, essential for cardiac muscle contraction.

• Pacemaker Cells: Specialized cardiac cells (e.g., SA node) that initiate and regulate the heartbeat.

• Contractile Cells: Cardiac muscle cells responsible for the contraction of the heart chambers.

EKG Waveforms and Intervals

The EKG waveform is a composite of all action potentials generated by pacemaker and contractile cells at a given time. The main components of a typical EKG trace are:

• P wave: Represents atrial depolarization (initiated by the SA node).

• QRS complex: Represents ventricular depolarization and atrial repolarization (the latter is masked by the large ventricular signal).

• T wave: Represents ventricular repolarization.

Intervals and Segments:

• P-R Interval: Time from the beginning of atrial excitation to the beginning of ventricular excitation (reflects conduction through the AV node).

• S-T Segment: Period when the entire ventricular myocardium is depolarized.

• Q-T Interval: Time from the beginning of ventricular depolarization to the end of ventricular repolarization.

Einthoven's Triangle and EKG Leads

Einthoven's Triangle is a conceptual model used to understand the placement of EKG leads on the body. It is formed by three limb leads (I, II, III) that record the heart's electrical activity from different angles.

• Lead I: Right arm to left arm

• Lead II: Right arm to left leg

• Lead III: Left arm to left leg

In clinical practice, more leads (5 or 12) are often used for greater sensitivity and to detect specific cardiac events.

Comparison: EKG vs. EMG

• EKG (Electrocardiogram): Measures electrical activity of the heart.

• EMG (Electromyogram): Measures electrical activity of skeletal muscles.

Cardiac Cycle

Overview of the Cardiac Cycle

The cardiac cycle refers to the sequence of mechanical and electrical events that repeat with every heartbeat, resulting in the coordinated contraction and relaxation of the atria and ventricles.

• Systole: Contraction phase of the heart chambers.

• Diastole: Relaxation phase of the heart chambers.

Chambers of the Heart

• Right Atrium and Left Atrium: Receive blood returning to the heart.

• Right Ventricle and Left Ventricle: Pump blood out of the heart.

• Interventricular Septum: Wall separating the right and left ventricles.

Blood Flow Through the Heart

• Into the Heart: Blood enters the right atrium via the superior and inferior vena cava, and the left atrium via the pulmonary veins.

• Out of the Heart: Blood leaves the right ventricle via the pulmonary artery (to the lungs) and the left ventricle via the aorta (to the body).

Heart Valves

Valves ensure unidirectional blood flow through the heart by opening and closing in response to pressure differences.

• Atrioventricular (AV) Valves: Prevent backflow into the atria when ventricles contract.

  • Tricuspid Valve: Right AV valve

  • Mitral (Bicuspid) Valve: Left AV valve

• Semilunar (SL) Valves: Prevent backflow into the ventricles when they relax.

  • Pulmonary Semilunar Valve: Right SL valve

  • Aortic Semilunar Valve: Left SL valve

Valve Operation Rules:

• Valves open when upstream pressure is greater than downstream pressure.

• Valves close when downstream pressure exceeds upstream pressure.

Phases of the Cardiac Cycle

1. Ventricular Filling:

   • Occurs during mid-to-late diastole.

   • AV valves are open; most blood flows passively into ventricles.

   • Atrial systole ("atrial kick") delivers the remaining blood.

   • End-Diastolic Volume (EDV): Volume of blood in each ventricle at the end of diastole.

2. Isovolumetric Contraction:

   • Atria repolarize and relax; ventricles begin to contract.

   • Ventricular pressure rises, closing AV valves.

   • All valves are closed; no blood is moving.

3. Ventricular Ejection (Emptying):

   • Ventricular pressure exceeds pressure in arteries, opening SL valves.

   • Blood is ejected; pressure in the left ventricle can reach ~120 mmHg.

   • End-Systolic Volume (ESV): Volume of blood remaining in the ventricle after systole.

4. Isovolumetric Relaxation:

   • Ventricles relax; pressure drops.

   • SL valves close; all valves are closed again.

   • When atrial pressure exceeds ventricular pressure, AV valves open and the cycle repeats.

Summary Table: Heart Valves and Their Function

Key Equations

• Ohm's Law (for electrical measurements):

• Stroke Volume (SV): The amount of blood ejected by a ventricle in one contraction.

• Cardiac Output (CO): The volume of blood pumped by each ventricle per minute.

Additional info: Cardiac output is a key measure of heart function, where HR is heart rate (beats per minute).